[en] The present study was undertaken to show the potential nephrotoxicity of tobramycin, given in two different dosage regimens, on the proximal convoluted tubules by using the light and transmission electron microscopic techniques. Thirty-five rats were divided into three groups: Group I served as control, Group II received tobramycin at 4mg/Kg of body weight intraperitoneally every 8 hours for ten days, and Group III received once-daily dosing of intraperitoneal injection of tobramycin at 12mg/Kg of body weight for ten days. The rats were sacrificed 3 days after the last injection. Small pieces of the right kidneys of all the animals were processed for light and electron microscopic examination. The study showed that tobramycin resulted in certain structural and ultrastructural changes in the proximal convoluted tubules. These changes included vacuolar degeneration in the epithelial cells, increased number of lysosomes with variably sized myeloid bodies, mitochondrial oedema, and loss of apical microvilli. These changes were clearly evident following multiple-daily dosing and were less obvious following once-daily dosing. Furthermore, regenerating tubular epithelial cells were evident following once-daily dosing administration. The experimental tobramycin toxicity can be reduced by administering equivalent amounts of the antibiotic in a once-daily dosing as opposed to multiple-daily injections. It is hoped that this study will contribute in the selection of a more appropriate dosing regimen for tobramycin in human beings. (author)